In hot roughing mills and plate mills, roller tables at either side of the mill stand are used to transport the product and to support the product during the rolling process.
In steel hot rolling, the roller tables usually use cylindrical rolls which support the product across its full width. But, in aluminium hot rolling, the material is much more easily damaged, scratched or stained by contact between the bottom surface of the product and the rolls of the tables. Also, in aluminium rolling, there is usually no surface treatment between hot and cold rolling and so the hot rolled surface quality has a major influence on the final product quality. In contrast, in steel rolling, there is usually a pickling treatment between hot and cold rolling to remove scale and clean the surface. Consequently, in the conventional aluminium hot rolling process, the product is normally only supported at the edges so that most of the bottom surface of the product is not in contact with the rolls.
Referring to FIGS. 1a and 1b, in order to ensure that the product P only contacts the rolls at the edges, hot aluminium mill roller tables 100 usually use either double tapered rolls 102a, 102b (FIG. 1a) or inclined half-width cylindrical rolls 104a, 104b (FIG. 1b). In the case of tapered rolls 102a, 102b, the taper sometimes has a compound form with different taper angles in different sections of the roll but the principle is still the same. The taper angle or inclination angle of the rolls 102a, 102b; 104a, 104b is typically between 1.3 and 3.6 degrees from the horizontal, depending on the final product thickness, width and strength at the rolling temperatures, as discussed in more detail later herein.
In most aluminium mills, tapered solid rolls, usually manufactured from single piece forgings, are used near to the mill stand because these rolls have to handle the impact forces and loads from the thick slabs. Further from the hot mill the product is thinner and hence half width inclined tubular cylindrical rolls are used.
However, there are a number of problems with using tapered or inclined cylindrical rolls. Referring now to FIG. 2, a problem that is common to the use of both tapered and inclined cylindrical rolls is that the pass-line height h1, h2 (i.e. the height of the bottom surface of the product above floor level, or datum D) varies with the width of the product P1, P2. This is an issue for the design of the other rolling mill equipment, such as the rolling mill stand and the shears, because different width products P1, P2 are delivered to the equipment at different pass-line heights h1, h2. Clearly, the larger the taper (or inclination) angle α of the rolls the greater is the variation of pass-line height with product width.
In addition to the pass-line height problem, tapered rolls suffer from problems due to the differences in roll peripheral (circumferential/surface) speed along the taper. One issue is that if the product is not on the center line, then the difference in speed at the two edges can cause the product to skew.
Inclined cylindrical half-width rolls do not have any problems with differences in peripheral speed along the roll but one of the issues with inclined cylindrical rolls is the drive mechanism. Referring again to FIG. 1b, the most common method for lighter duty roller tables 100 with tubular rolls is to use separate motors M1, M2 for each half-width roll 104a, 104b. However, clearly the cost of having two motors per roll instead of one is a significant disadvantage.
Another method is to group several half rolls together on each side by roller chains, toothed belts or gears and use one motor on each side per group. But all of these methods of driving multiple rollers from one motor suffer from reliability issues and hence mills generally prefer individually driven rollers.
According to CN201150936, another solution that has been used is to connect the two half width rolls via a drive coupling which will accommodate a small angle between the rolls so that only one drive per pair of half width rolls is required. A problem with this arrangement is that standard gear-type couplings are generally only suitable for small angles (typically 2 to 3 degrees across the joint—which implies that the each half roll can only have an angle relative to the horizontal of only 1 to 1.5 degrees). As discussed later herein, particularly for wider and thinner material, roll angles of only 1 to 1.5 degrees may not be sufficient. Other types of joints which can accommodate bigger angles (e.g. Hooke's type joints) could be used but these produce cyclical variations in the relative velocity of the two half rolls which is not desirable.
A further complication occurs in the case of so-called 1+1 mills. In a 1+1 or similar mill there is often a wide (typically 3 to 4 m wide but possibly wider) plate/roughing mill stand and one or more narrower (typically 2 to 3 m wide) finishing mill stands. This type of mill produces two different products: plate products and strip products. In both cases, the rolling process starts with cast and scalped slab which can be up to 800 mm thick. For the strip products, the roughing/plate mill stand rolls a transfer bar (typically 20 to 60 mm thick) which then gets transported to the finishing mill stand for further rolling in coil form. (“Transfer bar” is the name given to the partially rolled product which is transferred from the roughing mill to the finishing mill, i.e. the roughing mill rolls the slab down to 20 to 60 mm and then the finishing mill rolls it down to final thickness). For the plate products, the finish rolling is carried out in the roughing/plate stand and the plate product could be as thin as 10 mm or even thinner.
In the case of the strip product, the surface finish is extremely critical and any contact between the bottom surface of the transfer bar and the roller table would result in material being scrapped. Therefore, it is very important to ensure that the transfer bar is only supported at the edges.
The critical consideration is the amount of sag of the transfer bar across its width when it is supported at the edges. The amount of sag depends on the width, thickness, temperature and grade of the material. Furthermore, because aluminium is typically hot rolled at relatively high temperatures relative to the melting point, typically between 550 and 300 degrees Celsius, material creep increases the sagging of the product especially at the end of long transfer bars. Furthermore, other forces acting on the product, such as the forces from centring guides and the impact forces between the product head end and the roller table rolls, can also increase the sagging of the product locally.
To ensure that even the thinnest and widest transfer bars do not make contact with the rolls except at the edges, mill designers calculate the optimum taper or inclination angle for the particular product range of the mill. Typically, for transfer bars, the optimum angle is relatively large—up to around 3.6 degrees depending on the final product thickness, width and strength at the rolling temperatures. Very often the mill designer also specifies a minimum transfer bar thickness dependent on width to ensure that the sagging of the product is not sufficient to contact the roll surface either in the center or inboard of the edges. However, limiting the minimum transfer bar thickness for the wider products is not ideal because this increases the load and power required in the finishing mill stand.
Another issue is that if the products that are rolled are changed during the lifetime of a rolling mill, then the angle might not be sufficient in the future. Of course, the roller table could be designed with even larger taper or inclination angles so that even thinner and wider transfer bars could be rolled. But, large angles exacerbate the problems discussed earlier; variation in pass-line height with width, speed differentials for taper rolls and the difficulty of driving half-width inclined rollers with single motors per pair. Therefore, the angle is usually chosen to be large enough for the anticipated products but no larger.
A complication that arises with 1+1 mills is that the thinnest and widest plate products sag so much that they would make contact with the rolls inboard of the strip edge even if very steep angles were used because the material is so thin and wide that it cannot support itself from the edges only. This is because the thinnest plate products, e.g. 10 mm, have only about half the thickness of the thinnest transfer bars and are also much wider, e.g. 4 m instead of 2 m. Consequently, dedicated plate mills, which might be expected to use larger taper or inclination angles because of the thinner and wider product, actually use relatively small angles and some contact inboard of the strip edge on the thinner and wider products is accepted. The use of smaller angles minimizes the pass-line height differences; if large angles were used on a plate mill, the pass-line height variations could be very large because of the much wider range of widths that are rolled. Also, in the case of tapered rolls, the use of small angles ensures that the peripheral speed differences (e.g. between contact points in the center and the edge) are minimized and this minimizes scratching and damage to the bottom surface. So, a problem with a 1+1 mill is that if the optimum (large) angle is selected for rolling transfer bars, this would result in very large pass-line height variations for plate products (and large differences in peripheral speed for tapered rolls). Whereas, if the optimum (small) angle were selected for plate products, then the minimum transfer bar thickness that could be rolled without bottom surface contact would be significantly thicker than the optimum.
A solution proposed by CN102773269 is the use of separate, movable central rollers. The idea is that for thin and wide plate these central rollers are raised to support the plate so that it does not sag. However, this solution is not ideal because the small contact area of this central roller is highly likely to cause surface damage especially because that roller is not driven. It could be driven of course, but this would introduce even more complexity.
A further complication that arises with 1+1 mills is that they might have sections of roller table with two different widths; for example wide roller tables suitable for plate product either side of the roughing/plate stand and narrower tables close to the finishing stand. If these two roller tables have different angles (e.g. relatively steep angles for transfer bars on the narrow table and relatively shallow angles for the wide tables) then there will be a mismatch in pass-line height between the two sections of table depending on the product width.
JP H06 246324 A discloses a roller table apparatus for transporting a metallic product comprising first and second rolls, and outboard ends of the rolls being supported by respective outboard bearings and inboard ends of the rolls being supported by respective inboard bearings.
Furthermore, at least one adjuster is disclosed to displace the rolls so as to adjust an angle of inclination of each of the longitudinal axes of the rolls with respect to the other and thereby to adjust a pass-line height of the product relative to a datum.
In view of the above, it would be desirable to avoid marking, scratching, or staining of the bottom surface of the product, to minimize pass-line height variation and preferably peripheral speed differences, and to accommodate table rolls with different lengths while still maintaining the same profile of the roll top surface.